In optical lithography apparatus employing pulsed ultraviolet (UV) radiation, optical projection elements of the apparatus, commonly made from fused silica, usually progressively degrade as a result of interaction with the UV radiation being projected. The degradation is a result of multiphoton effects. Accordingly, the operating cost of the apparatus is strongly dependent, among other factors, on the rate at which this degradation progresses. The degradation rate of the optical elements depends, among other factors, on the peak intensity of ultraviolet radiation pulses incident thereon. Ultraviolet radiation pulses are typically delivered to the projection apparatus by a laser such as an excimer laser. One means for reducing the peak intensity of a radiation pulse delivered by the laser is to extend the duration of the optical pulse, i.e., to make the pulse longer. This is accomplished by an optical device known as a pulse extender or a pulse stretcher.
Commonly used prior-art pulse extenders utilize a partially reflective mirror (beamsplitter) to split the energy of a radiation pulse into two portions. One portion is transmitted through the beamsplitter as a first replica pulse, the other portion is delayed in an optical delay line. One portion of the delayed portion of the pulse (a second replica pulse) is spatially overlapped with the originally transmitted first replica of the pulse. Another portion of the delayed replica pulse is further delayed and portion of that portion (third replica) is temporally overlapped, and so on. Overlapped pulse replicas form the desired longer pulse. The longer pulse has the energy of the original pulse less any energy losses involved in splitting the original pulse and replicas thereof.
One example of prior-art pulse extender 20 is schematically illustrated in FIG. 1. Pulse extender 20 includes a beamsplitter 22 having a partially reflective and partially transmissive surface 24, and a delay loop 26 formed by concave spherical mirrors 28, 30, 32, and 34. The spherical mirrors relay an image of an incoming pulse (not shown) at the beamsplitter back onto the beamsplitter. A portion of a pulse incident on surface 24 of the beamsplitter is transmitted by the beamsplitter as indicated in FIG. 1 by a single arrowhead. The transmitted portion can be considered as the first replica pulse. Another part of the incident pulse reflected from surface 24, then sequentially from mirrors 28, 30, 32, and 34 as indicated in FIG. 1 by double arrowheads. Reflective surface of beamsplitter 24 reflects a portion of this delayed portion of the pulse (a second replica pulse) along the same path as the first-transmitted portion but delayed by a time τ, which is the round trip time in delay loop 26. Reflective surface of beamsplitter 24 transmits a portion of this replica pulse that undergoes further delays and division into reflected and transmitted portions. A subsequently delayed portion has lesser energy than a previously delayed portion. In theory at least, the number of round trips and replica pulses is infinite. In practice, however, the energy of replica pulses becomes vanishingly and uselessly small after as few as three replica pulses have been produced. An explanation of this operation is set forth below.
FIG. 2 schematically illustrates a generalized pulse extender 40 of the type illustrated in FIG. 1. A beamsplitter is represented by a single partially reflective partially transmissive surface 42, and a delay loop is represented by a rectangular path 44. Those skilled in the art will recognize that the delay loop 40 could include four mirrors arranged as illustrated in FIG. 1 or any other arrangement of the same or a different number of mirrors to accomplish a similar result.
The pulse extender in FIG. 2 can be analyzed, in one example, by considering a rectangular input pulse 46 of duration tp of about 20.0 nanosecond (ns) with beamsplitter 42 having a reflectivity RBS of about 64%. Optical losses L in loop 44, per round trip in the loop are assumed to be about 20%, optical delay τ in the delay loop is assumed to be equal to about 20.0 ns. In response to the single incident pulse 46, the output of extender 40 will include a sequence of pulses beginning with an originally transmitted (first) replica of the incident pulse 48 and increasingly delayed second and third replica pulses 50 and 52 respectively. Replica pulses 48, 50 and 52 would have an energy ratio 0.36:0.33:0.09. There would be subsequently delivered replica pulses albeit vanishingly small as noted above. The Time-Integral Square (TIS) pulse length is about 49.0 ns, providing a pulse-duration extension-ratio of 2.5. The TIS pulse length is the duration of a hypothetical rectangular pulse of equal energy that produces the same multiphoton effect as the pulse sequence. Total output energy in the pulse sequence is about 75% of the energy of input pulse 46. In a hypothetical case of zero losses in the delay loop, a maximum extension ratio of about 3.0 could be achieved.
It should be noted here that pulse replicas 48, 50, and 52 are depicted in FIG. 2 as having a rectangular temporal shape for convenience of illustration at the scale of the drawing. Those skilled in the art will be aware that in practice, such replicas would have a temporal shape closer to a Gaussian shape. FIG. 3 schematically depicts pulse replicas 48, 50, and 52 (dotted curves), represented as Gaussian-shaped pulses having a temporal width of about 20 ns measured at 50% of peak intensity. The pulses are temporally spaced or delayed, one from the next, by the temporal pulse width (τ) of 20 ns and have the energy ratio 0.36:0.33:0.09 exemplified above. The calculated sum of the replica pulses (the extended pulse) is depicted in FIG. 3 by solid curve 53. Noting, here, that the width of extended pulse 53 is also defined at the 50% peak intensity, it can be seen that the most delayed pulse replica 52 contributes little to extending the width of extended pulse 53.
Clearly, further pulse-width extension could be achieved by using two or more pulse width extenders 40 in series. This further extension, however, would be achieved at the expense of further loss of pulse energy and an increase in source power required to accommodate the extenders. Optical losses are due primarily to losses on each reflection from a mirror. By way of example, for commercially available mirrors, at ultraviolet wavelengths about 193 nanometers (nm) losses of about 4% can be expected at each mirror due to transmission, absorption, and scattering of incident radiation. For longer wavelengths smaller losses can be anticipated.
It is evident from the above-discussed analysis of a prior-art pulse extender that there is a limit to the achievable pulse extension by any one extender due to the energy of an extended pulse being concentrated in the first two replicas of the input pulse. There is a need for a pulse extender that provides a series of replica pulses of approximately equal magnitude.